feat(backup): add pre-migration backup, periodic backup, backfill warning, and backup management UI

Four improvements to the database backup mechanism:

1. Auto backup before schema migration - creates a snapshot when
   upgrading from an older database version, providing a safety net
   beyond the existing SAVEPOINT rollback mechanism.

2. Periodic startup backup - checks on app launch whether the latest
   backup is older than 24 hours and creates a new one if needed,
   ensuring all users have recent backups regardless of usage patterns.

3. Backfill failure notification - switch now returns SwitchResult with
   warnings instead of silently ignoring backfill errors, so users are
   informed when their manual config changes may not have been saved.

4. Backup management UI - new BackupListSection in Settings > Data
   Management showing all backup snapshots with restore capability,
   including a confirmation dialog and automatic safety backup before
   restore.

src/lib/query/mutations.ts

@@ -2,6 +2,7 @@ import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { useTranslation } from "react-i18next";
 import { toast } from "sonner";
 import { providersApi, settingsApi, type AppId } from "@/lib/api";
+import type { SwitchResult } from "@/lib/api/providers";
 import type { Provider, Settings } from "@/types";
 import { extractErrorMessage } from "@/utils/errorUtils";
 import { generateUUID } from "@/utils/uuid";
@@ -171,7 +172,7 @@ export const useSwitchProviderMutation = (appId: AppId) => {
   const { t } = useTranslation();
 
   return useMutation({
-    mutationFn: async (providerId: string) => {
+    mutationFn: async (providerId: string): Promise<SwitchResult> => {
       return await providersApi.switch(providerId, appId);
     },
     onSuccess: async () => {